This invention relates to a motor control circuit and method for regulating the torque of an induction motor, and more particularly to regulating the torque of an induction motor operated at a variable frequency using a modified in-phase component of stator current as a controlled variable.
Most traction applications specify precise regulation of motor torque so as to provide smooth, controlled acceleration. Smooth control of a subway car or a trolley car, for example, is needed for passenger comfort. In the application of ac induction motors to such applications in an adjustable speed, variable frequency drive system, precise control of torque is hindered by the fact that no single variable which is directly proportional to torque is readily measurable. The torque is proportional to the air gap flux per pole and to the in-phase rotor current, however the present invention relates to a technique that avoids the need for sensing the air gap flux.